The earliest rolling cutter earth boring bits had teeth machined integrally from steel, conically shaped, earth disintegrating cutters. These bits, commonly known as "steel-tooth" or "mill-tooth" bits, are typically used for penetrating relatively soft geological formations of the earth. The strength and fracture-toughness of steel teeth permits the effective use of relatively long teeth, which enables the aggressive gouging and scraping action that is advantageous for rapid penetration of soft formations with low compressive strengths.
However, it is rare that geological formations consist entirely of soft material with low compressive strength. Often, there are streaks of hard, abrasive materials that a steel tooth bit should penetrate economically without damage to the bit. Although steel teeth possess good strength, abrasion resistance is inadequate to permit continued rapid penetration of hard or abrasive streaks.
Consequently, it has been common in the art since at least the early 1930s to provide a layer of wear resistant metallurgical material called "hardfacing" over those portions of the teeth exposed to the severest wear. The hardfacing typically consists of extremely hard particles, such as sintered, cast or macrocrystalline tungsten carbide dispersed in a steel, cobalt or nickel alloy binder or matrix. Such hardfacing materials are applied by heating with a torch a tube of the particles which welds to the surface to be hardfaced a homogeneous dispersion of hard particles in the matrix. After hardfacing, the cone is preferably heat treated, which typically includes carburizing and quenching from a high temperature to harden the cone. The particles are much harder than the matrix but more brittle. After hardening, the matrix has a hardness preferably in the range from 53 to 68 Rockwell C (RC). The mixture of hard particles with a softer but tougher steel matrix is a synergistic combination that produces a good hardfacing.
There have been a variety of different hardfacing materials and patterns, including special tooth configurations, to improve wear resistance or provide self sharpening. Generally, the hardfacing applied to the teeth of new bits is in a preapplication ratio range of 50 to 80 percent carbide particles, typically about 70 percent, in a metal matrix of iron, nickel, cobalt or their alloys. The thickness of the hardfacing deposit on new bits is usually about 1/16 to 1/8 inch over the flanks, end portions and top of the crest of the tooth. Portions of the hardfacing may be somewhat thicker. The thicker portions are generally at the corners where the flanks intersect the crest. These thicker portions may be up to double that of other areas.
Worn bits have been retipped by adding a type of hardfacing to the teeth after they have been worn. Often a substantial part of the original hardfacing would be worn off along with a portion of the underlying steel teeth. The retipping hardfacing materials typically used are about 35-50% by weight of carbide particles with a fairly soft copper, bronze, brass or iron matrix. The soft matrix allows the retipper to shape the new tooth being formed. Depending on the extent of wear, the hardfacing may be quite thick, even greater than 3/16 inch on top of the top of the underlying steel tooth. Retippers normally do not heat treat the retipped bit. Because of the softer matrix and the lack of heat treating the hardness of the matrix after application on a retipped tooth would normally be considerably less than a new bit tooth. While satisfactory for very soft drilling, such as water well drilling, the retipped hardfacing is not as wear resistant as the original equipment hardfacings described above, which contain a higher percentage of carbide particles and a harder matrix metal.
While hardfacing provides good wear resistance for a steel tooth bit, teeth are still susceptible to breakage. Breakage is generally thought to occur due to portions of the teeth being too brittle. Brittleness, particularly in smaller diameter drill bits, is at least partially caused by the underlying carburized layer. The standard manufacturing procedure is to carburize the steel cone after it is hardfaced to harden the surface for resisting erosion. The carburizing is performed in a furnace, using either a gas or a pack process. This process adds carbon throughout the hardfacing, and also increases the carbon content in a carburized layer near the surface of the steel, the layer having a depth of about 0.030 to 0.140 inch depending upon bit size and application. The carburizing process creates a carburized layer even below the hardfacing.
If the tooth crest is fairly sharp as in smaller cones, the carburized layer becomes deeper at the crest of the tooth because the carburized layers on the two flanks and sharp crest tend to merge. This makes the crest brittle. Even though subsequently carburized, this brittle area can be subject to premature tooth failure.